Where did I go wrong in this code?

A few of my classes aren't passing automated tests. Unfortunately, said tests do not provide any useful information about why they failed. Here is my code for a couple of the classes. I'd really appreciate it if you could tell me where I went wrong. The comments should explain what each method is supposed to do.

public class CellPhone {
protected String ownerName;

public CellPhone(String owner) {
ownerName = owner;

}

public String receiveCall(CellPhone sender) {
// returns a String of the form:
// owner's name " is receiving a call from " sender's name
String receivingCall = ownerName + " is receiving a call from " + sender;

return receivingCall;
}

public String call(CellPhone receiver) {
// returns a String by using the receiver to invoke receiveCall
// while passing in the current phone
String invokingReceiveCall = receiver.receiveCall(receiver);